The invention relates to a method for determining the corrosion resistance of deep-drawable iron sheets for body parts of motor vehicles, and to an apparatus for performing the method.
Iron sheets manufactured by the steel industry are used on a large scale in the motor vehicle industry for the manufacture of body parts of motor vehicles due to good deep-drawing characteristics. Before being further processed into body parts of a wide variety of configurations, these iron sheets are generally subjected to a phosphating process and then to Painting. The painting operation serves primarily to protect the iron sheets from any subsequent corrosion, and/or to increase the corrosion resistance which naturally exists with this material.
Painting technology for iron sheets has developed a high technical standard particularly with regard to a uniform build-up of paint on the iron sheet. Nevertheless, widely varying behavior of the individual painted body parts as to their corrosion resistance is continually discovered. This varying corrosion resistance in the iron sheets used for the various body parts is attributable to differences in the characteristics of these iron sheets. These characteristics of the iron sheets are visible even through the paint coating of the sheets. Extremely thin oxide coatings on the surface of the sheets are responsible for the variable corrosion quality of the sheets. The thickness and the composition of these oxide layers substantially determine the corrosion resistance of the coated iron sheets.
Various test methods for determining the corrosion resistance of unpainted and painted iron sheets have been disclosed. These methods of determination the future corrosion resistance to be expected of such sheets however are generally very time-consuming. Open air weathering tests of painted sheets last up to twelve months, if these tests are to yield results with any reliability. Even time-accelerated tests under various climatic stresses, such as damp heat tests or salt water spray tests for example, last from several days to several weeks. Moreover, the result obtained from these tests cannot immediately be generalized, nor can it be transferred without distinction to sheets of a different type or from different manufacturers.
Electrochemically operating measuring probes for the determination of the rate of corrosion of metals in corrosive media are disclosed by German Patent Specifications Nos. (DE-PS) 2,030,671, (DE-PS) 2,033,619 and (DE-PS) 2,252,442. However, the application of these procedures to metal sheets presupposes a sampling. The samples require a highly time-consuming conditioning including a cleaning of the sample and also an equilibration so that the effective measuring time for each sample is approximately two hours. As a result, these procedures are not practicable in an on line processing arrangement.
German Patent Specification No. (DE-PS) 2,537,429 relates to a corrosion testing device which operates utilizing a sprayed salt solution. German Published, Unexamined Patent Application No. (DE-OS) 3,405,024 relates to a corrosion testing device which operates with a corrosion mist. However, these methods, in which corrosion of metal surfaces is therefore accelerated by stress climates, require even more time than many of the above-noted methods; up to three weeks in most cases. Moreover, the values obtained by these methods display a low correlation to the corrosion behavior found in practice.
German Published, Unexamined Patent Application (DE-OS) No. 3,418,066 describes a corrosion testing appliance for magnetizable steel sheet utilizing a magnet system which is passed across the steel sheet to create a magnetic flux in the steel sheet. However, this procedure only detects corrosion damage to the sheet which has already developed, and therefore cannot indicate the latent tendency to corrosion which already exists in iron sheets German Published, Unexamined Patent Application No. (DE-OS) 3,339,151 discloses a method and an apparatus for evaluating corrosion protection for a metallic surface, the metallic surface being provided with a surface coating and the extent of a cathodic disbonding being measured German Published, Unexamined Patent Application No. (DE-OS) 3,509,577 also discloses a method and apparatus for testing anti-corrosion layers on a structure stressed mechanically after completion, particularly on the body of a motor vehicle.
A common feature of all these determination methods and/or measuring devices is that, as already explained in detail in some cases, they are both complicated and time-consuming to perform and also involve a large and in some cases complicated apparatus structure.
It is therefore an object of the present invention to provide a method for the determination of the corrosion resistance of deep-drawable iron sheets for body parts of motor vehicles which is simple to perform and utilizes an uncomplicated apparatus technology that provides reproducible measurement results which are consistently obtained rapidly and reliably.
The method and apparatus for determining the corrosion resistance of deep-drawable iron sheets utilize a quantitative determination of the thickness and composition of the oxides on the surfaces of the sheets which, as explained above, substantially determine the corrosion resistance of an uncoated and also of a coated iron sheet.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.